The effects of Co₃O₄ on the structure and unusual magnetism of LaCoO₃

Bulk La_wCoO₃ particles with w = 1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La₂O₃ and Co₃O₄. The resulting particles are characterized as LaCoO₃ crystals interfaced with a crystalline Co₃O₄ phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO₃ phase resulting from the Co₃O₄ content, but magnetization data indicate that the amount of Co₃O₄ strongly affects the ferromagnetic ordering at the interfaces below T_C ≈ 89 K. In addition to ferromagnetic long-range order, LaCoO₃ exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe ≤ H ≤ 5 kOe is fit to a combination of a power law ((T - T_C)/T_C)^β behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent β = 0.63 ± 0.02 is consistent with 2D (surface) ordering. Increased Co₃O₄ correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ≈ 40 K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co₃O₄ interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.